A chartered plane specially equipped to contain infectious diseases took off at 1:12 a.m. local time from the airport in Liberia’s capital, Monrovia. An Associated Press reporter saw the four-vehicle convoy arrive at the airport.

Although hospital officials haven’t released the patient’s identity, the aid group she was working with has identified her as 59-year-old Nancy Writebol.

Writebol will be treated at a special isolation unit at Emory University Hospital in Atlanta. Dr. Kent Brantly was taken to the unit Saturday after arriving from Liberia aboard the same aircraft.

Brantly and Writebol contracted Ebola while treating patients at a missionary clinic in Liberia.

Both are being treated with an experimental drug never tested for safety in humans.